This invention relates to a bottom structure of a bottle-shaped container made of synthetic resin showing a polygonal transversal cross section. More particularly, it relates to a bottom structure of a bottle-shaped container made of synthetic resin showing a polygonal transversal cross section, and having a central bulged section formed at a center of the bottom and projecting into the container.
A bottle-shaped containers made of synthetic resin such as polyethyleneterephthalate is formed by biaxially-oriented blow-molding a preform. Such containers are widely used for the purpose of containing beverages. The container shows a transversal cross section that may be substantially circular, substantially square, substantially rectangular or of some other profile. The container is required to have a flat grounding portion at a bottom thereof so as to be able to stand on itself.
FIGS. 8–10 illustrate a known container 101 of the type under consideration that has a substantially rectangular transversal cross section. The container 101 comprises a neck 103, a body 105 and a bottom 107 connected to the body 105. The body 105 shows a substantially rectangular transversal cross section, and comprises two longer sides 151, 152 and two shorter sides 153, 154. The bottom 107 comprises a bottom wall 171 having a grounding edge 175 at a peripheral edge thereof, and a bottom peripheral wall 173 standing upwardly from the grounding edge 175. The bottom wall 171 is formed at a center thereof with a central bulged section 177 which is protruded into the container. The bottom wall 171 between the central bulged section 177 and the grounding edge 175 operates as a grounding portion 181. The grounding portion 181 is required to be flat, so that the container can stand on itself.
Meanwhile, in a case of a container showing a substantially circular transversal cross section, a preform is radially oriented to form the container. Since the preform is substantially uniformly oriented in all radial directions, the grounding portion of the container is uniformly oriented to be formed in all radial directions.
However, in a case of a container showing a substantially square or rectangular transversal cross section, an orientation magnification of the preform on a diagonal line is the largest, and an orientation magnification at a portion located off the diagonal line is smaller than that on the diagonal line. Thus, the portion of the container formed with such smaller orientation magnification tends to occur sink, so as to effect a moldability and the self-standing ability of the container.
In the case of the prior art container illustrated in FIGS. 8–10, the orientation magnification of the preform is the largest on the diagonal line L. To the contrary, the orientation magnification of the preform is the smallest on a center line M (passing through a center of each of the longer sides). As a result, sink tends to occur easily at a portion which nucleus is the center line M (shaded portion in FIG. 10), so as to consequently lose the flatness of the grounding portion and damage the self-standing ability of the container. Additionally, the orientation magnification of the preform on a center line N (passing through a center of each of the shorter sides) is smaller than the orientation magnification on the diagonal line L. Thus, sink tends to occur easily at a portion which nucleus is the center line N, compared with a portion on the diagonal line L.